Liquid crystal displays have brought to the world a low-power, flat-screen technology that has enabled many new portable devices to be produced. As the demand for more portable devices grows, so does the demand for liquid crystal displays with improved performance. Reflective displays are important not only because they reduce power consumption, but also because they can be clearly seen in sunlight. Back-lit or transmissive displays are necessary when the display will be used in a poorly lighted environment.
Displays with bistable memory are attractive for reducing power consumption and increasing battery life compared to displays in which the image needs to be continuously refreshed. Bistable liquid crystal displays were developed using cholesteric liquid crystalline materials. With a low concentration polymer network or with suitably prepared substrate surfaces, both the focal-conic and planar textures of a cholesteric liquid crystal can be made stable and it is possible to electrically switch between the two states.
Cholesteric displays that employ the polymer network may use cells with rubbed substrates to achieve a homogeneous alignment of the liquid crystal at the surface of the cell substrate. Homogeneous alignment tends to favor the planar texture. The polymer network or suitably prepared substrate surface prevents the focal-conic texture from transforming to the planar texture.
Reflective cholesteric liquid crystal displays are popular for portable signs as well as hand held devices where low-power consumption and hence long battery life is important. These reflective displays are readable in bright sunlight as well as room light. Bistable cholesteric reflective liquid crystal displays are addressed in two states, the reflecting planar texture and the weakly scattering focal conic texture. A black absorbing ink or paint layer is typically applied to the back substrate of the display and causes the focal conic texture to appear black, while light reflected by the planar texture is the color of its characteristic Bragg reflectance.
Under many lighting conditions back lighting is desirable over front lighting. One advantage of a back lit display is that light is more uniformly distributed over the display. In addition, back lighting sources are readily available. Prior art reflective cholesteric displays could only be front lit. U.S. Pat. No. 5,796,454 to Ma discloses one scheme that addresses back lighting. There is a need for a cholesteric liquid crystal display that can be selectively operated in a front lit mode as well as a back lit mode.